Substrate-induced structural modulation of a CoO(111) bilayer on Ir(100)

Abstract

We report on an unusual structural modulation of a single CoO͑111͒ bilayer grown on Ir͑100͒-͑1 ϫ 1͒ by oxidation of slightly less than one monolayer of Co deposited on the substrate. Quantitative low-energy electron diffraction and scanning tunneling microscopy in combination with standard x-ray photoelectron spectroscopy and thermal-desorption spectroscopy reveal a cobalt layer next to the substrate covered by an oxygen layer. Both layers' hexagonal atomic arrangements are, however, strongly distorted by the quadratic substrate and form a c͑10ϫ 2͒ superstructure on that. The Co layer's buckling amplitudes and atomic bond lengths to Ir atoms are consistent with the hard-sphere radius of metallic Co. The oxide's binding to the substrate appears to be further characterized by two types of oxygen ions. One of them is close to the expected rocksalt-type stacking with respect to the cobalt layer while the other type resides nearly on top of Ir atoms. Its hard-sphere radius is only 0.77 Å ͑in contrast to 1.25 Å in the CoO bulk͒ and it is by about 1 Å closer to the substrate than the other type. Being so almost coplanar with the Co layer, it locally forms a hexagonal boron-nitride-type oxide. The oxygen bond to Ir can be interpreted as local pinning of the oxide to the substrate so modulating the entire oxide bilayer.

Incommensurate Moiré overlayer with strong local binding: CoO(111) bilayer on Ir(100)

“…27 Testing a range of U parameters, a value of U −J=1 eV was found to give the best agreement between the present calculations and experiment. 19 This choice of U −J=1 eV is also supported by the calculated bulk volume, which is very close to experiment, and by a satisfactory description of the electronic structure as compared to a more sophisticated, but very costly, approach using hybrid functionals. 28 Hence, if not stated otherwise all results of the present work refer therefore to this choice of U −J=1 eV.…”

“…19 we first discuss the c(10 × 2) phase in the claimed MT configuration. We performed DFT calculations using the standard PBE functional with two distinct starting configurations: in one case we started with a slightly distorted, but otherwise flat, CoO bilayer that should easily converge into the structure found on Pd(100).…”

“…We performed DFT calculations using the standard PBE functional with two distinct starting configurations: in one case we started with a slightly distorted, but otherwise flat, CoO bilayer that should easily converge into the structure found on Pd(100). 20 In the other case, we modified the above structure to contain two different types of oxygen species (low-lying and high-lying ones mimicking the LEED results 19 ) as indicated by the different shading in Fig. 1.…”

“…Candidates are rock-salt type FeO, MnO, and CoO films of (111) orientation, for which films of ultimate thinness -single hexagonal metal-oxygen bilayers -have been found on a number of metal substrates. [11][12][13][14][15][16][17][18][19][20] In these cases high-order commensurate phases were reported with unit cell dimensions of about ten times that of the substrate, corresponding to about 10% lattice misfits between substrates and oxide films. In the case of (100) oriented substrates, this just leads to close correspondence of row distances in one direction and, by a slight uniaxial distortion, the films achieve one-dimensional commensurability (row-on-row matching).…”

“…1) as suggested by experiment. 18,19 We imposed mirror symmetry in the [011] direction. In the first configuration, termed "mirror top" (MT), the mirror plane contains one Co, one O, and one Ir atom of the first layer ( Fig.…”